1. Field of the Invention
The present invention generally relates to an optical pickup apparatus and an identification apparatus for identifying the type of optical record medium used for the optical pickup apparatus, and more particularly to an optical pickup apparatus for reproducing record information of optical record media of various types, which substrates have various thicknesses, which distances from their surfaces to their information record planes are different from each other or which information pits on record tracks have various sizes, and an identification apparatus for identifying the type of such an optical record medium.
2. Description of the Related Arts
There is a multi-disc player, which can reproduce a CD (Compact Disc) and a LD (Laser vision Disc) etc., as a reproducing apparatus able to reproduce a plurality of types of optical record media.
In this kind of multi-disc player, in order to realize an optical system optimum for each of the optical discs, which thicknesses, refraction coefficients, etc., of their disc protection layers or their disc substrates are different from each other, a plurality of optical pickup apparatuses, each of which is exclusive for one of the optical discs of various types, are provided and are selectively used in accordance with the loaded optical disc to be reproduced. Thus, the optimum reproduction operation can be performed with respect to each of various types of optical discs by one multi-disc player.
In this kind of multi-disc player, in case of identifying the types of optical discs which disc thicknesses are different from each other, a mechanical switch is provided so as to identify the type of the loaded optical disc on the basis of the fact that this mechanical switch is pressed or not by the loaded optical disc. Further, in case of identifying the types of optical discs which external shapes e.g. the external diameters are different from each other, an optical detection device such as a light sensor is equipped to detect the existence and non-existence of the reflection light reflected by the loaded optical disc.
However, since a plurality of optical pickup apparatuses, each of which is exclusive for one of the optical discs of various types, are equipped in the above mentioned multi-disc player, the size and cost of the apparatus is increased, which is a serious problem in the practical sense.
In order to overcome this problem, it may be proposed to use one optical pickup apparatus commonly for a plurality of optical discs of various types. However, once the thickness of the protection layer (substrate) is far off from the optimum thickness set for each of the optical pickup apparatus, a spherical aberration is generated in the focused light beam as the reading light beam at the information record plane of the optical disc, which is another serious problem to perform a precise reproduction operation.
More concretely, assuming that the wavelength of the reading light beam emitted from a laser diode is .lambda., the refraction coefficient of the protection layer of the optical disc is n, the difference between the actual thickness of the protection layer of the optical disc and the optimum thickness of the protection layer set for the optical pickup apparatus is .DELTA.d, and the numerical aperture of the objective lens is NA, the spherical aberration W.sub.40 is expressed by a following expression (1). EQU W.sub.40 ={(n.sub.2 -1)/8n.sup.3 }*.DELTA.d*(NA.sup.4 /.lambda.) (1)
Thus, as the difference .DELTA.d is increased, the spherical aberration W.sub.40 is also increased and the signal quality of the read out signal of the optical pickup apparatus is degraded, which is the problem.
Further, the above mentioned mechanical or optical identification apparatus for identifying the type of the loaded optical disc cannot identify the type of the loaded optical disc, if the thicknesses and refraction coefficients of the optical discs are substantially same to each other but only the distances from the surfaces of the protection layers to the information record planes are different from each other. In this case, a complicated identifying operation such as a comparison of record formats etc., will be necessary.
More concretely, those identification apparatuses cannot differentiate a first optical disc of both sides recording type produced by bonding two discs each of which has a protection layer with 0.6 mm thickness, from a second optical disc of one side recording type, which has an external diameter same as the first optical disc and which has a protection layer with 1.2 mm thickness, which is the problem.
By the way, in the above mentioned optical pickup apparatus, the spot diameter of the reading light beam is set to be a value optimum for the size of the information pit of the optical record medium to be reproduced.
The spot diameter R of the reading light beam is proportional to the numerical aperture NA of the objective lens and the wavelength .lambda. of the reading light beam. Namely, the spot diameter satisfies a following expression (2). EQU R.varies..lambda./NA (2)
Therefore, in case of reproducing the optical record medium which has an information pit of relatively small size, in order to make the spot diameter R of the reading light beam smaller correspondingly, the numerical aperture NA of the objective lens is increased assuming that the wavelength .lambda. of the reading light beam is constant.
In other words, assuming that the wavelength .lambda. of the reading light beam is constant, as the numerical aperture NA is increased, the smaller information pit can be reproduced, while, as the numerical aperture NA is decreased, the larger information pit can be reproduced.
The numerical aperture NA of the objective lens has a peculiar value for each objective lens. Thus, the optical pickup apparatus, which is adjusted to be optimum with respect to a first information pit having a certain size, is not suitable for a second information pit having a size different from that of the first information pit. Therefore, the distortion will be generated in the reproduction signal if this optical pickup apparatus is used with respect to an optical record medium formed with the second information pit, which is a problem in this case.
In this manner, it is not possible for one optical pickup apparatus to reproduce optical discs which sizes of the information pits are different from each other. Even if the reproduction is performed in such a condition, the precise reproduction operation is not possible, which is the problem.